Numerous processes are known for the separation of air by cryogenic distillation into its constituent components; nitrogen, oxygen, and argon, the latter being an inert gas which occurs to the extent of 0.93% by volume in the atmosphere. The use of argon has become increasingly important in various industrial and commercial operations, but is somewhat limited by the relatively high cost of its production in pure form. Among the known methods, in addition to cryogenic separation, are recovery from residual purge gas in the ammonia synthesis process, and by the treatment of atmospheric nitrogen with certain metals to form nitrites, leaving the unreacted argon.
Argon is typically produced at a purity of 95% to 98% from a cryogenic air separation plant with residual oxygen and nitrogen impurities. These impurities are usually removed by deoxygenation with hydrogen as a catalyst and subsequent cryogenic distillation to remove N.sub.2 and H.sub.2. Recovery of argon from a single-column cryogenic nitrogen plant heretofore has not been effective. Economics have dictated that argon cannot be recovered from cryogenic nitrogen plants, which do not additionally produce high-purity oxygen. Argon is present as the minor component in air and has a volatility between oxygen and nitrogen and is thus very difficult to produce without the simultaneous separation of pure oxygen.
Presently, argon is not recovered from cryogenic air separation plants adapted for nitrogen production, e.g., those that produce nitrogen for enhanced oil recovery applications, or gas inerting, or electronics processing. Today argon is primarily seen as a by-product of large scale plants.
It would be most useful to be able to modify a standard, single-column nitrogen generator process, so that it would foster the co-production of crude argon, and with only modest added power consumption. Existing technology, as mentioned, can effectively purify crude argon for commercial uses.
High purity gaseous nitrogen is produced directly by well-known cryogenic separation methods. Liquid nitrogen is typically produced by initially producing gaseous nitrogen in a cryogenic air separation unit, (ASU) and subsequently treating the gaseous nitrogen in a liquefier. Modified forms of cryogenic ASU's have been developed to directly produce liquid nitrogen. See U.S. Pat. No. 4,152,130 which discloses a method of producing liquid oxygen and/or liquid nitrogen.
The production of medium purity nitrogen or oxygen by non-cryogenic air separation processes is also achieved by using absorption, adsorption. and membrane-based processes, see U.S. Pat. No. 4,230,463.
Incorporating one or more membrane units into a cryogenic cycle at some point, or points, can improve the efficiency and economics of the system. Specifically, incorporating a membrane unit allows a given cryogenic cycle to be operated efficiently at conditions that might be inefficient when operated on a stand-alone basis. Consequently, cryogenic cycles that were considered inefficient in the past may, when properly coupled with a membrane system, yield superior and feasible processes.
One known process involves separating components of a gas stream by integrating a cryogenic separation unit and a membrane separation unit. U.S. Pat. No. 4,595,405 teaches generation of one or more nitrogen-rich streams in the gas or liquid stage, or both from a feed stream comprising air. The patent does not suggest how an air feed stream could be economically treated to produce crude argon by integrating a cryogenic process system with a membrane separation unit, as well as producing nitrogen.
It is therefore, an object of the present invention to permit the recovery of argon from single-column cryogenic nitrogen plants which do not also produce high-purity oxygen. It is another object to make a useful application of the known fact that a build-up of argon concentration relative to nitrogen and oxygen occurs in the middle portion of a high-pressure cryogenic nitrogen production column.
A still further object of the invention is to provide means to modify a single column nitrogen process cycle so that it will utilize argon build-up therein and allow co-production of useful volumes of crude argon. These and other objects, aspects, and features of the invention will become apparent to one skilled in the art from the specification, claims and drawings appended hereto.